The present invention is directed, in general, to circuit boards and, more specifically, to a multi-purpose finish for printed wiring boards (PWBs) and method of manufacture of such PWBs.
Printed Wiring Board (PWB) manufacturing processes are changing at a rapid rate because of the increasing demand for enhanced performance. The demand for enhanced performance is due to higher circuit densities, an increase in board complexities and an increase in the cost of environmental compliance. Many types of final finishes are used on PWBs. Final finish selection is generally dependent on the requirements the board must ultimately meet. Surface circuits on PWBs usually include copper and copper alloy materials that should be coated to provide good mechanical and electrical connection with other devices in the assembly.
Typically, the coating on the circuits is called the surface finish. The circuits include non-contact areas and contact areas. The finish applied to the non-contact areas is called non-contact finish and the finish applied to the contact areas is called contact finish. The non-contact areas include wirebonding areas, chip attach areas, soldering areas and other non-contact areas. Both non-contact and contact finishes must meet certain different requirements. Non-contact finish requirements include good solderability, good wirebonding performance (for some PWBS, depending on applications), and high corrosion resistance. Contact finish requirements on the other hand, include high conductivity, high wear resistance and high corrosion resistance. To meet these different requirements, different coatings have been used for non-contact finishes and contact finishes.
Some typical non-contact finishes include hot air solder level (HASL) coating, electroless nickel coating with immersion gold on top (EN/IAu), organic solderability preservative (OSP) coating, and organo-metallic, such as organo-silver, solderability preservative (OSP/Ag). A typical contact finish may include an electrolytic nickel coating with an electrolytic hard gold layer (gold-nickel or gold-cobalt alloys with nickel or cobalt less than 0.3 wt.%) on top. To coat any of the above non-contact finishes on the circuits and to coat the finish on the contact areas requires considerable processing steps (24-28 steps), and a great amount of time, all of which decreases production yields and increases cost.
The most common non-contact finish process in circuit boards is HASL. The use of HASL over the last few years has decreased dramatically. This is primarily due to the increasing demand for boards employable in mixed technologies, including surface mount technologies, of which HASL is not. In manufacturing a PWB using HASL, a photoresist layer is placed on a base copper laminate substrate, and subjected to ultraviolet light, which develops the pattern for the desired circuits. Next, the photoresist is washed, leaving openings in the areas that were masked off from the UV light, in the pattern of the circuits on the copper laminate. Electrolytic copper (acid copper) is then plated on top of the base copper laminate on the circuit areas, through the openings in the photoresist. After applying the electrolytic copper, a layer of tin or tin-lead, is pattern plated on the circuits to act as an etch resist. The remaining photoresist material is stripped from the substrate, exposing the base copper laminate. The base copper laminate is then removed, exposing the bare substrate (commonly glass enforced plastics). At this point in the HASL process, nothing remains on the substrate except the etch resist material plated on the copper circuit lines.
One disadvantage of this process is that extra-processing steps have to be applied. The etch resist material has to first be applied and then removed, and usually solder mask (SM) applied before HASL finishing can be accomplished. Furthermore, HASL coatings cannot be employed in the applications of mixed technologies due to their inability to meet substrate coplanarity and wire bonding requirements. The HASL finishing is a labor and maintenance intensive process and causes many environmental concerns because it usually contains lead.
The EN/IAu process requires essentially the same steps, up through the removal of tin or tin-lead and applying of SM, as the HASL process. Then, instead of applying flux and undergoing HASL, plating of electroless nickel and immersion gold is performed on the circuits. As with the HASL process, the EN/Au process has also many disadvantages. It suffers from high porosity, many processing steps, degradation when exposed to soldering temperatures and high expense.
There are many other competing technologies that are applied, but all require an etch resist to first be applied and then removed, demanding many processing steps. OSP and OSP/Ag are two of these post etch technologies applied directly to the surface of copper circuits. However, they have limited storage lives, degrade when exposed to soldering temperature and do not have good wire bondability.
None of the above-mentioned non-contact finishes can be used as a contact finish due to their poor wear resistence.
A contact finish, such as an electrolytic nickel layer with an electrolytic hard gold on top, must be applied on the contact areas after the non-contact finish is coated (as suggested previously). The contact finish has good conductivity and high wear resistance. However, it cannot be used as a non-contact finish due to its poor solderability and wire bondability. And to apply the contact finish to the board requires another masking step where SM is applied, the contact finish plated, then the SM stripped off. This increases processing steps and decreases production yields due to mis-registration of the SM.
The PWB industry has recently started to evaluate alternative surface finishes. There is a high demand for a multi-purpose finish that can be used for both non-contact circuits and contact areas and that can replace tin-lead in the PWB finishing process, to lower environmental concerns and make electronic circuit manufacturing xe2x80x9cgreener.xe2x80x9d To be qualified, such a finish should provide high etch resistance, good solderability, good wirebonding performance, high conductivity, high wear resistance, high corrosion resistance/low porosity, coplanarity (uniform thickness distribution), integrity after exposure to soldering temperatures for up to 10 minutes, ability to integrate into present manufacturing stream, long storage life (6 to 12 months), favorable economics and environmental safety. With such a finish, one can dramatically reduce the processing steps, increase the production throughput, reduce the cost and improve the quality of the PWBs. It was previously thought that palladium or palladium-nickel alloys might be used as a multi-purpose finish in PWBs. However, pure palladium suffers from relatively high porosity and relatively high cost, and palladium-nickel alloys exhibit lower wear resistance than what is needed for today""s demanding technologies.
Accordingly, what is needed in the art is a multi-purpose finish that meets all the above-mentioned requirements and achieves all the above-mentioned objectives.
To address the above-discussed deficiencies of the prior art, the present invention provides a multi-purpose finish for a PWB and a method of manufacturing the same. In one embodiment, the PWB includes: (1) a substrate having a conductive trace located thereon and (2) a multi-purpose finish including palladium alloy where palladium is alloyed with cobalt or a platinum group metal and is located on at least a portion of the conductive trace, which forms both a non-contact finish and a contact finish for the PWB. Platinum group metal is a metal selected from the group consisting of Ruthenium, Rhodium, Palladium, Rhenium, Osmium, Iridium and Platinum.
The present invention therefore introduces the broad concept of employing a palladium alloy, where the palladium alloy is alloyed with cobalt or a platinum group metal, as a multi-purpose finish for PWBs, suitable for forming both non-contact and contact finishes. The palladium alloy when alloyed with cobalt includes about 1 wt.% or more of cobalt and may further include nickel or iron. The palladium alloy may further be alloyed with both cobalt and a platinum group metal. The palladium alloy acts also as an etch resist layer.
In one embodiment, the non-contact finish coats at least a portion of a non-contact area. In a related embodiment, the contact finish coats at least a portion of a contact area. In another embodiment the palladium alloy is located on all of the conductive traces. The conductive traces preferably comprise copper. However, other conductive materials such as copper alloys, aluminum, nickel, silver, gold, platinum or their alloys may be used for the conductive traces.
The palladium alloy employed in the present invention is such that palladium comprises a weight percent of the palladium alloy ranging from about 10 wt.% to about 95 wt.% and cobalt or a platinum group metal comprises a weight percent of the palladium alloy ranging from about 5 wt.% to about 90 wt.% of the palladium alloy.
The PWB may also preferably include a material formed over the multi-purpose finish, and in such instances, the material may be palladium, silver, gold, rhodium, ruthenium, platinum, tin or their alloys, or an organic solderability preservative (OSP).
In another embodiment, nickel may be applied, using conventional deposition processes, under the palladium alloy, forming a nickel under-layer. Those skilled in the art should understand that the nickel deposition step may be optional and omitted in some applications. However, where nickel is required it should also be understood that other materials such as nickel alloy, cobalt, cobalt alloy, iron or iron alloy may also be used.
In another embodiment of the present invention, the non-contact areas comprise surface-mount pads, wire bond pads, solder pads or interconnections. Interconnections as used by Applicants are defined as the circuit traces, plated through holes (PTH) and micro-vias. Those skilled in the pertinent art will realize, however, that the palladium alloy may be advantageously employed in other PWB finishes, such as for vias or decorative designs.
The substrate may comprise various materials that are typically used for PWB fabrication. Examples of these materials contain epoxies, polyimides, fluorinated polymers, ceramics, polyesters, phenolics, and aramide paper.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention are described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.